SNEmulDS is the best, followed by SnezziDS, but SnezziDS does require a slot-2 device with GBA compatibility in order to run. SNES DS actually has better compatibility than most give it credit for, but it requires you to build the rom a special way in order to maximize the compatibility, and the tools to build it have disappeared from the main site.

Speed usually isn't too much of an issue with SNEmulDS, it's the graphical problems that will get to you. This is a trade-off of the way it emulates the games. It uses the DS's dedicated 2D hardware to greatly speed up the rendering time (down to about 5% of the CPU time) but this causes potential graphical glitches. A full software renderer like the ones for the PSP would be much more accurate graphically but would lag like crazy due to the increased CPU power needed.

Just make sure you get either v0.5 Beta or v0.6 Alpha. For some reason, the blog on the main site reverted back to a time when v0.3 was the latest, but the download link for v0.6 Alpha still remains on the sidebar. Very strange.